Arterial blood pressure (BP) exerts a critical influence on physiological homeostasis in humans, and abnormalities in BP regulation have potential pathophysiological consequences. The autonomic nervous system (ANS) is a key regulator of the cardiovascular determinants of BP. There may be age-related changes in ANS support of arterial blood pressure, and these may be at least partially related to differences in adrenergic sensitivity. The general aims of this study are to test the hypotheses that: 1) basal ANS tone makes a significant contribution to the maintenance of resting BP and its circulatory determinants in the healthy adult human; 2) ANS supoort of BP is increased with primary (healthy) aging; 3) adrenergic sensitivity is reduced with primary aging, and 4) regular exercise alters the ANS-cardiovascular integration. To test these hypotheses we are using the autonomic ganglionic antagonist, Trimethaphan, to eliminate ANS input to cardiovascular tissues, and thereby quantify its contribution to the maintenance of BP. The adrenergic agonists pheynlephrine (alpha-agonist), and isproterenol (beta-agonist) are infused during measurements of heart rate, BP and calf blood flow, to determine adrenergic sensitivity (i.e., effector tissue responsiveness to adrenergic stimuli). Four groups of healthy subjects are being studied; 10 young sedentary, 10 young endurance exercise-trained, 10 older sedentary, and 10 older endurance exercise-trained. Preliminary results indicate the ANS support of basal BP is greater in older than young sedentary adults, and that alpha-adrenergic sensitivity is enhanced with exercise training.